It is known that the biological treatment, for example, of water, consists of breaking down organic impurities through the action of a free or fixed purifying biomass housing various microorganisms; bacteria, yeast, protozoans, metazoans, etc. In the free biomass process, using activated sludge, it is impossible to concentrate a great number of different species of microorganisms, which are difficult to decant to the extent that the concentration of the biomass is formed by decanting. The process is thus limited in terms of the applicable load in BOD (biological oxygen demand) and COD (chemical oxygen demand). In a fixed biomass system, the biomass (with bacteria) is concentrated using a collection medium. Suitability for decantation in this case is no longer of vital importance, and the purifying potential of this technique is far superior to the conventional processes.
Among the most efficient processes based on the fixed biomass purification principle, those patented and developed by the present inventor can be cited, including the "Biocarbone" (registered trademark) process, and the technique of using a granular bed composed of two areas having different granulometric and biological characteristics in one ascending water current reactor (French Pat. No. 76.21246 published under No. 2 358 362; No. 78.30282 published under No. 2 439 749; No. 86.13675 published under No. 2 604 990).
In the "free biomass" techniques, in this case, we will refer primarily to fluidized bed processes, wherein products having a density less than 1 are used for the biofilter material, such as, for example, expanded polymers, according to processes now in the public domain (French Pat. No. 1 363 510 of 1963; English Pat. No. 1 034 076 of 1962) whose various embodiments have yielded numerous patents (French Pat. Nos. 2 330 652; No. 2 406 664; No. 2 538 800; U.S. Pat. No. 4,256,573; Japanese Pat. No. 58-153 590, etc.).
The implementation of said floating materials and fluidized granular beds is advantageous in and of itself but involves certain problems and often presents disadvantages, several of which were brought to light by lengthy tests conducted by the present inventor. For example, in a biofilter with ascending water current on balls or granules less dense than water, if air is injected into the base of the filtering bed, filtration cycle lengths are unacceptable and the surface layer is quickly consolidated by suspended materials blocking the passage of the air bubbles; in this case, frequent washings are necessary. Moreover, when materials heavier than water (sand or similar materials) are fluidized, a considerable energy supply is required to pump the liquid, and it is difficult to keep the material inside the reactor. To correct this energy consumption problem, it was proposed to use a fluidized bed of light materials with air intake at the base of the bed, but with a descending water feed (U.S. Pat. No. 4,256,573 and Japanese Pat. No. 58.153590 cited above). However, beginning at certain descending water speeds, air bubbles become trapped inside the material or are carried by the flux of liquid, and the reactor cannot be aerated properly.
To remedy the aforementioned problems, the present inventor conducted extensive experiments in order to use all the advantages of a floating or fluidized bed, while attempting to eliminate the phenomena of the trapping of bubbles at the surface, the consolidation of the bed, energy expenditures, problems in washing the filtering bed, etc.